The present invention relates generally to materials for use in transistors, and more particularly to polarization-induced bulk doping in electronic materials exhibiting strong spontaneous and/or piezoelectric polarization (e.g., ferroelectric and/or piezoelectric materials), and the use of these materials in forming high electron mobility field-effect transistor devices.
Prior to the present invention, bulk doping in electronic materials has been achieved by impurity doping, which is well characterized by a shallow “hydrogenic” model. The carrier concentration and transport properties are determined by temperature, dopant concentration and scattering mechanisms including impurity scattering and phonon scattering etc. Therefore, the carrier mobility always suffers from impurity scattering and the carrier concentration decreases as temperature decreases. A good understanding on this matter led to the concept of modulation doping, which improved low temperature carrier mobilities in quantum-confined structures by many orders of magnitude.
In recent years, III-V Nitrides have emerged as important materials for high-power microwave electronic applications. [See, e.g., Mishra et al., Proceedings of the IEEE 90, 1022 (2002); L. F. Eastman and U. K. Mishra, IEEE Spectrum 39, 28 (2002)]. In particular, crystals such as III-Nitrides, Zinc Oxide and Lithium Niobate, etc., exhibit large embedded electronic polarization fields owing to the lack of inversion symmetry in the crystal structure. This implies there exists a dipole in each unit cell of the crystal. For a homogeneous bulk crystal surface, dipoles inside the crystal cancel and leave net opposite charges on the opposing crystal surface, which is characterized by spontaneous polarization. Dipoles can also be created when a crystal is under strain, characterized by piezoelectric polarization. Both spontaneous polarization and piezoelectric polarization have been exploited for applications in communications, radar, infrared imaging, memories, integrated optics etc.
In the most successful Nitride electronic devices, high-electron mobility transistors (HEMTs), the strong spontaneous and piezoelectric polarization fields in AlGaN and GaN have been used to make nominally un-doped two-dimensional electron gases (2DEGs) in AlGaN/GaN heterostructures. These devices have yielded excellent power and efficiency performance at microwave frequencies.
In addition to high power and efficiency, devices for many wireless applications are also required to have high linearity at microwave frequencies. Design of such linear devices for large-signal operation needs tailoring of the transconductance (gm) profile over the input gate voltage (Vg) range. However, the structure of traditional AlGaN/GaN HEMT-like devices does not lead itself to easy modification of the gm-Vgs profile. It has been shown that the gm-Vgs curve of metal semiconductor field effect transistors (MESFETs) can be tailored by designing the channel doping profile. [See, R. E. Williams and D. W. Shaw, IEEE Trans. Electron Device, ED-25, 600-605 (1978); R. Pucel, Electron. Lett., 14, 204 (1978); J. A. Higgins and R. L. Kuvas, IEEE Trans. Microwave Theory Tech., MTT-28, 9 (1980)]. GaN MESFETs therefore remain attractive for high-linearity microwave power applications. However, the device designer is constrained in their choice of channel charge due to gate leakage, breakdown and impurity scattering limited mobility.
Accordingly, it can be seen that there is a need for improved materials and devices, such as HEMT devices, for use in microwave power and other applications. Such materials and devices should overcome some or all of the above deficiencies and constraints.